The modeling and simulation of electric machines is highly desirable for numerous reasons, including evaluation of machine behavior in open-loop operation, analysis of complex drive systems which incorporate such machinery and the design of applicable feedback controllers and control algorithms. Off-line and real-time simulations are particularly useful in the evaluation of induction machines which, because of their rugged design, are often used in industrial applications and may be interfaced with physical signals, systems and subsystems.
Historically, system designers have directed their modeling efforts to analog simulations of synchronous machines. See for example, U.S. Pat. Nos. 4,945,508, 4,293,923 and 3,723,718 issued to Benejean, Jasmin et al. and Jaffe et al, respectively, each of which uses the modeling approach of analog simulation wherein complex analog circuitry, e.g., oscillators, a plurality of multipliers, phase-locked loop circuits, and various op amp based circuits are employed in the simulating device. As those skilled in the art will recognize, these prior art devices have proven difficult and expensive to build and are generally inflexible to adapt to machines of different ratings and parameters. They are also highly unreliable due to the large number of electronic components used and are relatively limited in ranges and modes of operation due to the inherent possibilities of saturation of op-amps.
Against this background, those skilled in the art will further recognize that even beyond these limitations, the prior art modeling approaches are generally inapplicable to induction machines because of the fundamental differences in the design, operation and behavior between synchronous and induction machines. For example, synchronous machines are known to have explicit field windings carrying DC current and damper windings on the rotor. Induction machines have no corresponding windings. Similarly, synchronous machines rotate at zero slip or "synchronous" speed. In contrast, induction machines always have non-zero slip. Thus, the mathematical model governing synchronous machines have terms relating to field current and induced current in damper windings, while the equations of induction machines have terms dependent on slip or slip speed.
These and other differences known to those skilled in the art render historical modeling approaches of the type referenced in the prior art inapplicable to induction machines.